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Improving the control structure of a high pressure leaching process

Knoblauch, Pieter Daniel (2015-03)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The main purpose of the base metal refinery (BMR) as operated by Lonmin at their Western Platinum Ltd BMR, is to remove base metals – such as copper and nickel – from a platinum group metal (PGM) containing matte. The leaching processes in which this is done pose several challenges to the control of the process. The most significant of these is the slow dynamics of the process, due to large process units, as well as the continuously changing composition of the first stage leach residue, which is not measured on-line. This is aggravated by the fact that the exact leaching kinetics (and therefore the effect of the disturbances) are not understood well fundamentally. The slow process dynamics mean that controllers cannot be tuned aggressively, resulting in slow control action. The large residence times and off-line composition analyses of major controlled variables also mean that the effects of operator set point changes are visible only the following day, often by a different shift of operators.
Dorfling (2012) recently developed a fundamental dynamic model of the pressure leach process at Lonmin‟s BMR. This dynamic model incorporates 21 chemical reactions, as well as mass and energy balances, into a system of 217 differential equations. The model provides a simulation framework within which improved control strategies can be investigated.
The primary aims of this study are twofold. The first is to validate the model for the purpose of the investigation and development of control structure improvements. This is done by comparing the model to plant data, and adapting it if necessary. The second aim to reconsider the current control philosophy to the extent that is allowed by the model‟s determined validity.
The current plant control philosophy aims to maintain a PGM grade of 65%, while the copper in the solids products of the second and third leaching stages should be below 25% and 3.5% by mass, respectively. Two areas of particular concern in this process that have been raised by Lonmin are the control of the temperature of the first compartment and the addition of pure sulphuric acid to control the acid concentration in the second stage leach.
Dynamic plant data were used to calibrate the model, which was migrated from its received MATLAB platform to Simulink, to assist with control development. Flow rates were imported from the data, with some data values adapted for this purpose, due to mass balance inconsistencies. The outputs from the calibrated model were compared with corresponding data values. The model was found to be suitable for the investigation and development of the control structures of pressure, temperatures and inventories (termed basic regulatory control) and the acid concentration and solids fraction in the preparation tanks (termed compositional regulatory control). It was, however, found to be inadequate for the investigation and development of supervisory control, since it does not provide accurate compositional results. The leaching of copper is especially under-predicted, with the predicted copper concentration in the second stage product being approximately 46% lower than data values.
The basic and compositional regulatory control structures were investigated. For each of these a base case was developed which aimed to represent the relevant current control structure, assuming optimal tuning. The variable pairings for the basic regulatory control were reconsidered using a method proposed by Luyben and Luyben (1997), since this part of the process does not permit the generation of a relative gain array (RGA) for variable pairing. The resulting pairing corresponds with Lonmin‟s current practice. Considering the temperature control of compartment 1, it was found that the addition of feed-forward control to the feedback control of the level of the flash tank improves the temperature control. More specifically, during an evaluation where the temperature‟s set point is varied up to 1%, the IAE of the temperature of compartment 1 was decreased with 7.5% from the base case, without disturbing the flash tank. The addition of feed-forward control allows for more rapid control and more aggressive tuning of this temperature, removing the current limit on ratio between the flash recycle stream and the autoclave feed.
The compositional control was investigated for the second stage leach only, due to insufficient flow rate and compositional information around the third stage preparation tank. Variable pairing showed that three additive streams are available for the preparation tanks of the second and third stage leach to control the acid concentration and solids fraction in those tanks. Focussing on the second stage, the aim was to determine whether the acid concentration in the flash tank can be successfully controlled without the addition of pure acid to the tank. With four streams available around the second stage preparation tank to control its mass/level, the acid concentration and solids fraction, three manipulated variables were derived from these streams. The resulting pairings were affirmed by an RGA. Control loops for the control of acid concentration and solids fraction in the flash tank were added as cascade controllers, using the preparation tank‟s control as secondary loops. The added compositional control was evaluated in two tests. The first of these entailed the adding of typical disturbances, being the flash recycle rate, the solids and water in the feed to the second stage preparation tank and the acid concentration in copper spent electrolyte. In the second test the control system was tested for tracking an acid concentration set point. It was found that the cascade structure controls the acid concentration in the flash tank less tightly than the base case (with an IAE that is 124% and 80.6% higher for the two tests), but that it decreases the variation of solids fraction (lowering the IAE with 40.8% with the first test) in the same tank and of the temperature in the first compartment (lowering the IAE with 73.6% in the second test). It is recommended that the relative effects of these three variables on leaching behaviour should be investigated with an improved model that is proven to accurately predict leaching reactions in the autoclave.